Editor’s Note: Today’s caption is the answer to Earth Observatory’s November puzzler.

About 290 million years ago, two large asteroids smashed into
Earth. The massive craters they left behind—now lakes—are still visible
from space. Astronauts have noticed and photographed the craters for decades, and the Operational Land Imager
on Landsat 8 captured a new image of Clearwater Lakes (Lac à l‘Eau
Claire) on June 29, 2013. The small inset box toward the lower left
marks the area shown in the lower image.

When they struck, the binary asteroids crashed into a part of Earth’s
crust that was fairly close to the equator. Since then, millions of
years of plate tectonics
have pushed the craters north into what is now northwestern Quebec. A
mere 20,000 years ago (not long from a geologic perspective), massive
sheets of ice covered northwestern Quebec, much like ice now covers Antarctica
and Greenland. During that period, ice sheets repeatedly advanced and
retreated, scouring the land of soil and rock during cool periods, and
then carving deep channels and rinsing the landscape with melt water
during warmer periods.

In the images above, the impact of that glacial scouring is evident.
The area has little topsoil and supports only sparse vegetation such as
spruce forests, grasses, and mosses. The erosion was so complete that
many land surfaces were scraped down to underlying bedrock, exposing
some of the oldest rocks in the world.
Meanwhile, the melt water from retreating glaciers left a dense network
of linear lakes and streams that now dominate the surface. Their
ubiquity and complexity is a sign that the geologically young drainage
network has yet to carve out its most efficient route to the sea.

While simple craters
have a circular, bowl-shaped form, large asteroids leave more complex
shapes. Clearwater East and Clearwater West, for instance, are both complex craters
with distinct central peaks. These peaks are caused by the
gravitational collapse of crater walls and subsequent rebound of the
compressed crater floor. Lake water covers the central peak of
Clearwater East, but bathymetric surveys of the lake floor confirm the
presence of a peak in its center.

The central peak on Clearwater West went through another geologic
twist. The central peak became so tall that it collapsed, producing a
feature known as a peak ring. The top of the peak ring rises well above
the modern water line, leaving the ring of islands seen above.

NASA Earth Observatory images by Jesse Allen and Robert Simmon, using Landsat 8 data from the USGS Earth Explorer. Caption by Adam Voiland. Congratulations to Allen Pope (NSIDC), D Arseneault, and Lachezar Filchev
for coming close to solving the puzzler and for providing interesting
background information about the area.

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